Differentiation is made between access networks and transport networks for the transmission of data. The access networks, which ensure the access to the consumer, are characterised by complex network structures. The system diversity of these primarily service-specific access systems is very great. Within a communications network, the access network has the function of enabling the subscribers to access switching devices, for example servers. From his location, each subscriber must firstly be connected to a supply node in order to avail himself of the offered services. All information technology means are basically suitable for the connection of the subscribers. In the long-distance network, the routing of the telecommunications traffic is concentrated in point-to-point connections between the different network and switching nodes. On the other hand, in the access network very different requirements must be fulfilled. Standardised functions must be implemented for the technical cooperation between the subscriber exchange and the terminals. The subscribers themselves are not homogeneously distributed over the entire access area; rather, there are local areas with high subscriber densities as well as areas in which the subscribers occur only sporadically. On the basis of the relevant communications requirements, it is possible to distinguish between highly differing subscriber groups, such as private customers and smaller or larger business customers to whom appropriate transmission-technology solutions must be offered. It is in the case of the access networks that willingness is required on the part of the network operator to invest a large capital outlay. The components are exposed to environmental influences and must each be made available individually to the access network subscribers.
A method for constructing access network is known form the prior art German application 199 04 940.8. A transmission system of this kind for coded optical signals consists of optical transmission lines, optical splitters and optionally optical amplifiers insert to transmit code multiplexed optical signals. Each transmitter contains a coder in which the signal to be transmitted are coded prior to their transmission into the optical transmission network. The coding takes place optically by frequency coding using an optical filter. Each receiver, which wishes to receive the data per specific transmitter, must contain a decoder, which is tuned to the coder of this specific transmitter. In the simplest case the frequency ranges which are conductive for an optical signal and the frequency ranges which are blocked for optical signals on the same in the coder and in the decoder. The code division multiplex methods are particularly as suitable for the transmission of not too high bit rates. The code division multiplex message is suitable for use as multipoint- to- multipoint network such as for example a LAN. An example for a broadcasting network the unpublished German application 100 51 633.5. All this examples for access network require optical coding means with high performance. To achieve the coding effect especially optical cavities of Fabry-Perot types are used. To achieve the tunability phase shifters in the cavity of the optical coding means are needed. All effects that are utilized in refractive optical switches are commonly suitable for generating the additional phase shift in a cavity. In particular these are as follows:    a) The electrical optical effect that for example occurs in lithium niobate, plastic or III/V semiconductors for example InGaAsP/InP or GaAlAs/GaAs and whereby an electrical field produces a change in refractive index.    b) Charge carrier injections into III/V semiconductors for example InGaAsP/InP or GaAlAs/GaAs wherein the change in refractive index is defined by the strength of the injection and    c) The thermo-optical effect that for example occurs in material systems of glass and silicon and where the temperature T determines the change in refractive index.
Electro-optical phase shifters are polarization dependent and induce high losses. The thermo-optical effect in fibers or wave-guide optics is not very reliable or too expensive due to the fact that the devices consume a high electrical power. Also known are mechanical influences on fibers or waveguide by stretching the wave-guides. Which is also known from prior art are liquid crystal phase shifters that are easy to build and consume not too much electrical power. It is known from the U.S. Pat. No. 6,154,591 to use a tunable optical device comprising a substrate and a superstrate first in the second optical wave guide sandwiched between that substrate and that superstrate so as to define the base between that wave guides an optical resonant cavity extending across that space, that space containing a liquid crystal material to commit tuning of the cavity and alignment means to orientate the liquid crystal material so that it responds to an applied electrical field wherein the alignment means is exposed on at least one of the substrate and the superstrate. Normally the polarization dependent due to the anisotropy structure of the liquid crystal molecules induces the strong birefringence. To avoid this polarization dependence it is also known to use a symmetric arrangement of liquid crystal structures to make a tunable devices polarization insensitive.